Various motor controller circuit configurations are known. One such configuration is the H-bridge or full bridge configuration in which four transistors are configured in an H pattern with the motor coil coupled to form the bridge of the H configuration. The transistor switches are controlled in pairs such that when a first pair of switches conduct, a first voltage signal is provided to the motor coil to cause a current to flow in a first direction through the coil, and when the second pair of switches conduct, a second voltage signal is provided to the motor coil to cause the current to flow through the coil in the opposite direction. The rate of turning on and off the transistor pairs controls the speed of the motor. The voltage signal provided by the motor driver circuit to the motor coil is referred to herein as the motor signal.
The speed of the motor may be determined from a rotor commutation signal that is generated by converting the magnetic field generated by a rotating motor element, such as an alternating pole ring magnet, to an electrical signal with the use of a magnetic field-to-voltage transducer, such as a Hall effect element. The output signal of the Hall effect element has a voltage proportional to the magnetic field and can be processed to generate a pulse train commutation signal having a period proportional to the motor speed.
Generally, a motor is started by the motor signal having a 100% duty cycle in order to achieve a predetermined motor speed at the fastest rate possible. The duty cycle of the motor signal can then be reduced from the 100% duty cycle to a lesser duty cycle in order for the motor speed to be maintained at the predetermined motor speed. In one particular example of a motor, a single-phase brushless motor, a 100% duty cycle is generated when one transistor pair is conducting 50% of the time and the other transistor pair is conducting the other 50% of the time.